CY8C28xxx PSoC Programmable System-on-Chip TRM, Document No. 001-52594 Rev. *G
381
Digital Blocks
This resynchronization process (forcing the state back to
idle) occurs regardless of the value of the STOP bit sample.
It is important to reset as soon as possible, so that maximum
performance can be achieved.
shows an
example where the RX block clock bit rate is slower than the
external TX bit rate. The sample point shifts to successively
later times. In the extreme case shown, the RX samples the
STOP bit at the trailing edge. In this case, the receiver has
counted 9.5 bit times, while the transmitter has counted 10
bit times. Therefore, for a 10-bit message, the maximum
theoretical clock offset, for the message to be received cor-
rectly, is represented by one-half bit time or five percent. If
the RX and TX clocks exceed this offset, a logic 0 may be
sampled for the STOP bit. In this case, the Framing Error
status is set.
Figure 17-38. Example RX Resynchronization
This theoretical maximum will be degraded by the resyn-
chronization time, which is fixed at approximately 42 ns. In
a typical 115.2 Kbaud example, the bit time is 8.70
s. In this
case the new maximum offset is:
((4.35 ms – 42 ns) / 4.35 ms) x 5% or 4.95%
At slower baud rates, this value gets closer to the theoretical
maximum of five percent.
Status Generation.
There are five status bits in a Receiver
block: RX Reg Full, RX Active, Framing Error, Overrun, and
Parity Error. All status bits, except RX Active and Overrun,
are set synchronously on the STOP bit sample point.
RX Reg Full indicates a byte has been received and trans-
ferred into the RX Buffer register. This status bit is cleared
when the user reads the RX Buffer register (DR2). The set-
ting of this bit is synchronized to the STOP sample point.
This is the earliest point at which the Framing Error status
can be set; and therefore, error status is defined to be valid
when RX Reg Full is set.
RX Active can be polled to determine if a reception is in
progress. This bit is set on START detection and cleared on
STOP detection. This bit is not
and there is no way
for the user to clear it.
Framing Error status indicates that the STOP bit associated
with a given byte was not received correctly (expecting a '1',
but received a '0'). This will typically occur when the differ-
ence between the baud rates of the transmitter and receiver
is greater than the maximum allowed.
Overrun occurs when there is a received data byte in the RX
Buffer register and a new byte is loaded into the RX Buffer
register, before the user has had a chance to read the previ-
ous one. Because the RX Buffer register is actually a latch,
Overrun status is set one-half cycle before RX Reg Full.
This means that although the new data is not available, the
previous data has been overwritten because the latch was
opened.
Parity Error status indicates that resulting parity calculation
on the received byte does not match the value of the parity
bit that was transmitted. This status is set on the sample
point of the STOP signal.
Start
1
1
0
1
0
0
1
0
1
RXD
Stop
Start
RX clock is slower than TX clock.
Stop Bit is just
recognized.
Need to re-sync
as soon as
possible.
Any delay in
re-sync will cut
into the optimal
sync of the next
byte.
Sample points are
successively later
in the bit times.
Содержание CY8C28 series
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Страница 85: ...84 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Internal Main Oscillator IMO ...
Страница 93: ...92 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G External Crystal Oscillator ECO ...
Страница 97: ...96 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Phase Locked Loop PLL ...
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Страница 425: ...424 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Analog Reference ...
Страница 461: ...460 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Two Column Limited Analog System ...
Страница 477: ...476 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Digital Clocks ...
Страница 483: ...482 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Multiply Accumulate MAC ...
Страница 513: ...512 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Internal Voltage Reference ...
Страница 523: ...522 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Switch Mode Pump SMP ...
Страница 533: ...532 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G I O Analog Multiplexer ...
Страница 537: ...536 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Real Time Clock RTC ...
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